D2D operation method performed by terminal in wireless communication system and terminal using same

ABSTRACT

Provided are a device-to-device (D2D) operation method and device which are performed by a terminal in a wireless communication system. The method comprises: receiving D2D configuration information indicating a plurality of resources which can be used for a D2D operation from a network; selecting a specific resource from among the plurality of resources; and performing the D2D operation with another terminal using the selected specific resource, wherein the specific resource is selected on the basis of a reference signal received power (RSRP) which the terminal has received from the network.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/113,028, filed on Jul. 20, 2016, U.S. Pat. No. 10,251,160, which theNational Stage filing under 35 U.S.C. 371 of International ApplicationNo. PCT/KR2015/001076, filed on Feb. 2, 2015, which claims the benefitof U.S. Provisional Application No. 61/934,642, filed on Jan. 31, 2014and 61/935,849, filed on Feb. 5, 2014, the contents of which are allhereby incorporated by reference herein in their entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to wireless communications, and moreparticularly, to a method for a device-to-device (D2D) operationperformed by a terminal in a wireless communication system, and theterminal using the method.

Related Art

In International Telecommunication Union Radio communication sector(ITU-R), a standardization task for International MobileTelecommunication (IMT)-Advanced, that is, the next-generation mobilecommunication system since the third generation, is in progress.IMT-Advanced sets its goal to support Internet Protocol (IP)-basedmultimedia services at a data transfer rate of 1 Gbps in the stop andslow-speed moving state and of 100 Mbps in the fast-speed moving state.

For example, 3rd Generation Partnership Project (3GPP) is a systemstandard to satisfy the requirements of IMT-Advanced and is preparingfor LTE-Advanced improved from Long Term Evolution (LTE) based onOrthogonal Frequency Division Multiple Access (OFDMA)/SingleCarrier-Frequency Division Multiple Access (SC-FDMA) transmissionschemes. LTE-Advanced is one of strong candidates for IMT-Advanced.

There is a growing interest in a Device-to-Device (D2D) technology inwhich devices perform direct communication. In particular, D2D has beenin the spotlight as a communication technology for a public safetynetwork. A commercial communication network is rapidly changing to LTE,but the current public safety network is basically based on the 2Gtechnology in terms of a collision problem with existing communicationstandards and a cost. Such a technology gap and a need for improvedservices are leading to efforts to improve the public safety network.

The public safety network has higher service requirements (reliabilityand security) than the commercial communication network. In particular,if coverage of cellular communication is not affected or available, thepublic safety network also requires direct communication betweendevices, that is, D2D operation.

D2D operation may have various advantages in that it is communicationbetween devices in proximity. For example, D2D UE has a high transferrate and a low delay and may perform data communication. Furthermore, inD2D operation, traffic concentrated on a base station can bedistributed. If D2D UE plays the role of a relay, it may also play therole of extending coverage of a base station.

There is a need to regulate which scheme a network configures resourcesused to perform a D2D operation and how to use configured resources toperform the D2D operation by a terminal.

SUMMARY OF THE INVENTION

The present invention provides a method for a device-to-device (D2D)operation performed by a terminal in a wireless communication system,and the terminal using the method.

In one aspect, provided is a device-to-device (D2D) operation methodperformed by a first terminal in a wireless communication system. TheD2D operation method includes receiving D2D configuration informationfrom a plurality of resources which can be used for a D2D operation froma network, selecting a specific resource from the plurality of resourcesand performing the D2D operation with another terminal using theselected specific resource. The specific resource is selected based on areference signal received power (RSRP) which the terminal has receivedfrom the network.

In another aspect, provided is a terminal for performing a D2D operationin a wireless communication system. The terminal includes a RF unitconfigured to send and receive radio signals and a processor connectedto the RF unit to be operated. The processor receives D2D configurationinformation from a plurality of resources which can be used for a D2Doperation from a network, selects a specific resource from the pluralityof resources and performs the D2D operation with another terminal usingthe selected specific resource. The specific resource is selected basedon a reference signal received power (RSRP) which the terminal hasreceived from the network.

The UE may select a suitable D2D resource from a plurality of D2Dresources based on reception power of a reference signal. That is, theUE may recognize a location thereof related to coverage of a cell toselect a corresponding D2D resource.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a wireless communication system to which the presentinvention is applied.

FIG. 2 is a diagram showing a wireless protocol architecture for a userplane.

FIG. 3 is a diagram showing a wireless protocol architecture for acontrol plane.

FIG. 4 is a flowchart illustrating the operation of UE in the RRC idlestate.

FIG. 5 is a flowchart illustrating a process of establishing RRCconnection.

FIG. 6 is a flowchart illustrating an RRC connection reconfigurationprocess.

FIG. 7 is a diagram illustrating an RRC connection re-establishmentprocedure.

FIG. 8 illustrates substates which may be owned by UE in the RRC_IDLEstate and a substate transition process.

FIG. 9 shows a basic structure for ProSe.

FIG. 10 shows the deployment examples of types of UE performing ProSedirect communication and cell coverage.

FIG. 11 shows a user plane protocol stack for ProSe directcommunication.

FIG. 12 shows the PC 5 interface for D2D direct discovery.

FIG. 13 is an embodiment of a ProSe discovery process.

FIG. 14 is another embodiment of a ProSe discovery process.

FIG. 15 illustrates a UE-NW relay.

FIG. 16 illustrates a UE-UE relay.

FIG. 17 illustrates a D2D operation method performed by a UE accordingto an embodiment of the present invention.

FIG. 18 illustrates cell coverage according to the range of the RSRP.

FIG. 19 illustrates the relationship between a range of an RSRP and atier and the relationship between the tier and a cell.

FIG. 20 illustrates an example of applying a method described withreference to FIG. 17 to FIG. 19.

FIG. 21 is a block diagram illustrating a terminal according to anembodiment of the present invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1 shows a wireless communication system to which the presentinvention is applied. The wireless communication system may also bereferred to as an evolved-UMTS terrestrial radio access network(E-UTRAN) or a long term evolution (LTE)/LTE-A system.

The E-UTRAN includes at least one base station (BS) 20 which provides acontrol plane and a user plane to a user equipment (UE) 10. The UE 10may be fixed or mobile, and may be referred to as another terminology,such as a mobile station (MS), a user terminal (UT), a subscriberstation (SS), a mobile terminal (MT), a wireless device, etc. The BS 20is generally a fixed station that communicates with the UE 10 and may bereferred to as another terminology, such as an evolved node-B (eNB), abase transceiver system (BTS), an access point, etc.

The BSs 20 are interconnected by means of an X2 interface. The BSs 20are also connected by means of an S1 interface to an evolved packet core(EPC) 30, more specifically, to a mobility management entity (MME)through S1-MME and to a serving gateway (S-GW) through S1-U.

The EPC 30 includes an MME, an S-GW, and a packet data network-gateway(P-GW). The MME has access information of the UE or capabilityinformation of the UE, and such information is generally used formobility management of the UE. The S-GW is a gateway having an E-UTRANas an end point. The P-GW is a gateway having a PDN as an end point.

Layers of a radio interface protocol between the UE and the network canbe classified into a first layer (L1), a second layer (L2), and a thirdlayer (L3) based on the lower three layers of the open systeminterconnection (OSI) model that is well-known in the communicationsystem. Among them, a physical (PHY) layer belonging to the first layerprovides an information transfer service by using a physical channel,and a radio resource control (RRC) layer belonging to the third layerserves to control a radio resource between the UE and the network. Forthis, the RRC layer exchanges an RRC message between the UE and the BS.

FIG. 2 is a diagram showing a wireless protocol architecture for a userplane. FIG. 3 is a diagram showing a wireless protocol architecture fora control plane. The user plane is a protocol stack for user datatransmission. The control plane is a protocol stack for control signaltransmission.

Referring to FIGS. 2 and 3, a PHY layer provides an upper layer with aninformation transfer service through a physical channel. The PHY layeris connected to a medium access control (MAC) layer which is an upperlayer of the PHY layer through a transport channel. Data is transferredbetween the MAC layer and the PHY layer through the transport channel.The transport channel is classified according to how and with whatcharacteristics data is transferred through a radio interface.

Data is moved between different PHY layers, that is, the PHY layers of atransmitter and a receiver, through a physical channel. The physicalchannel may be modulated according to an Orthogonal Frequency DivisionMultiplexing (OFDM) scheme, and use the time and frequency as radioresources.

The functions of the MAC layer include mapping between a logical channeland a transport channel and multiplexing and demultiplexing to atransport block that is provided through a physical channel on thetransport channel of a MAC Service Data Unit (SDU) that belongs to alogical channel. The MAC layer provides service to a Radio Link Control(RLC) layer through the logical channel.

The functions of the RLC layer include the concatenation, segmentation,and reassembly of an RLC SDU. In order to guarantee various types ofQuality of Service (QoS) required by a Radio Bearer (RB), the RLC layerprovides three types of operation mode: Transparent Mode (TM),Unacknowledged Mode (UM), and Acknowledged Mode (AM). AM RLC provideserror correction through an Automatic Repeat Request (ARQ).

The RRC layer is defined only on the control plane. The RRC layer isrelated to the configuration, reconfiguration, and release of radiobearers, and is responsible for control of logical channels, transportchannels, and PHY channels. An RB means a logical route that is providedby the first layer (PHY layer) and the second layers (MAC layer, the RLClayer, and the PDCP layer) in order to transfer data between UE and anetwork.

The function of a Packet Data Convergence Protocol (PDCP) layer on theuser plane includes the transfer of user data and header compression andciphering. The function of the PDCP layer on the user plane furtherincludes the transfer and encryption/integrity protection of controlplane data.

What an RB is configured means a process of defining the characteristicsof a wireless protocol layer and channels in order to provide specificservice and configuring each detailed parameter and operating method. AnRB can be divided into two types of a Signaling RB (SRB) and a Data RB(DRB). The SRB is used as a passage through which an RRC message istransmitted on the control plane, and the DRB is used as a passagethrough which user data is transmitted on the user plane.

If RRC connection is established between the RRC layer of UE and the RRClayer of an E-UTRAN, the UE is in the RRC connected state. If not, theUE is in the RRC idle state.

A downlink transport channel through which data is transmitted from anetwork to UE includes a broadcast channel (BCH) through which systeminformation is transmitted and a downlink shared channel (SCH) throughwhich user traffic or control messages are transmitted. Traffic or acontrol message for downlink multicast or broadcast service may betransmitted through the downlink SCH, or may be transmitted through anadditional downlink multicast channel (MCH). Meanwhile, an uplinktransport channel through which data is transmitted from UE to a networkincludes a random access channel (RACH) through which an initial controlmessage is transmitted and an uplink shared channel (SCH) through whichuser traffic or control messages are transmitted.

Logical channels that are placed over the transport channel and that aremapped to the transport channel include a broadcast control channel(BCCH), a paging control channel (PCCH), a common control channel(CCCH), a multicast control channel (MCCH), and a multicast trafficchannel (MTCH).

The physical channel includes several OFDM symbols in the time domainand several subcarriers in the frequency domain. One subframe includes aplurality of OFDM symbols in the time domain. An RB is a resourcesallocation unit, and includes a plurality of OFDM symbols and aplurality of subcarriers. Furthermore, each subframe may use specificsubcarriers of specific OFDM symbols (e.g., the first OFDM symbol) ofthe corresponding subframe for a physical downlink control channel(PDCCH), that is, an L1/L2 control channel. A Transmission Time Interval(TTI) is a unit time for subframe transmission.

The RRC state of UE and an RRC connection method are described below.

The RRC state means whether or not the RRC layer of UE is logicallyconnected to the RRC layer of the E-UTRAN. A case where the RRC layer ofUE is logically connected to the RRC layer of the E-UTRAN is referred toas an RRC connected state. A case where the RRC layer of UE is notlogically connected to the RRC layer of the E-UTRAN is referred to as anRRC idle state. The E-UTRAN may check the existence of corresponding UEin the RRC connected state in each cell because the UE has RRCconnection, so the UE may be effectively controlled. In contrast, theE-UTRAN is unable to check UE in the RRC idle state, and a Core Network(CN) manages UE in the RRC idle state in each tracking area, that is,the unit of an area greater than a cell. That is, the existence ornon-existence of UE in the RRC idle state is checked only for each largearea. Accordingly, the UE needs to shift to the RRC connected state inorder to be provided with common mobile communication service, such asvoice or data.

When a user first powers UE, the UE first searches for a proper cell andremains in the RRC idle state in the corresponding cell. The UE in theRRC idle state establishes RRC connection with an E-UTRAN through an RRCconnection procedure when it is necessary to set up the RRC connection,and shifts to the RRC connected state. A case where UE in the RRC idlestate needs to set up RRC connection includes several cases. Forexample, the cases may include a need to send uplink data for a reason,such as a call attempt by a user, and to send a response message as aresponse to a paging message received from an E-UTRAN.

A Non-Access Stratum (NAS) layer placed over the RRC layer performsfunctions, such as session management and mobility management.

In the NAS layer, in order to manage the mobility of UE, two types ofstates: EPS Mobility Management-REGISTERED (EMM-REGISTERED) andEMM-DEREGISTERED are defined. The two states are applied to UE and theMME. UE is initially in the EMM-DEREGISTERED state. In order to access anetwork, the UE performs a process of registering it with thecorresponding network through an initial attach procedure. If the attachprocedure is successfully performed, the UE and the MME become theEMM-REGISTERED state.

In order to manage signaling connection between UE and the EPC, twotypes of states: an EPS Connection Management (ECM)-IDLE state and anECM-CONNECTED state are defined. The two states are applied to UE andthe MME. When the UE in the ECM-IDLE state establishes RRC connectionwith the E-UTRAN, the UE becomes the ECM-CONNECTED state. The MME in theECM-IDLE state becomes the ECM-CONNECTED state when it establishes S1connection with the E-UTRAN. When the UE is in the ECM-IDLE state, theE-UTRAN does not have information about the context of the UE.Accordingly, the UE in the ECM-IDLE state performs procedures related toUE-based mobility, such as cell selection or cell reselection, without aneed to receive a command from a network. In contrast, when the UE is inthe ECM-CONNECTED state, the mobility of the UE is managed in responseto a command from a network. If the location of the UE in the ECM-IDLEstate is different from a location known to the network, the UE informsthe network of its corresponding location through a tracking area updateprocedure.

System information is described below.

System information includes essential information that needs to be knownby UE in order for the UE to access a BS. Accordingly, the UE needs tohave received all pieces of system information before accessing the BS,and needs to always have the up-to-date system information. Furthermore,the BS periodically transmits the system information because the systeminformation is information that needs to be known by all UEs within onecell. The system information is divided into a Master Information Block(MIB) and a plurality of System Information Blocks (SIBs).

The MIB may include the limited number of parameters which are the mostessential and are most frequently transmitted in order to obtain otherinformation from a cell. UE first discovers an MIB after downlinksynchronization. The MIB may include information, such as a downlinkchannel bandwidth, a PHICH configuration, an SFN supportingsynchronization and operating as a timing reference, and an eNBtransmission antenna configuration. The MIB may be broadcasted on a BCH.

SystemInformationBlockType1 (SIB1) of included SIBs is included in a“SystemInformationBlockType1” message and transmitted. Other SIBs otherthan the SIB1 are included in a system information message andtransmitted. The mapping of the SIBs to the system information messagemay be flexibly configured by a scheduling information list parameterincluded in the SIB1. In this case, each SIB is included in a singlesystem information message. Only SIBs having the same schedulingrequired value (e.g. period) may be mapped to the same systeminformation message. Furthermore, SystemInformationBlockType2 (SIB2) isalways mapped to a system information message corresponding to the firstentry within the system information message list of a schedulinginformation list. A plurality of system information messages may betransmitted within the same period. The SIB1 and all of the systeminformation messages are transmitted on a DL-SCH.

In addition to broadcast transmission, in the E-UTRAN, the SIB1 may bechannel-dedicated signaling including a parameter set to have the samevalue as an existing set value. In this case, the SIB1 may be includedin an RRC connection re-establishment message and transmitted.

The SIB1 includes information related to UE cell access and defines thescheduling of other SIBs. The SIB1 may include information related tothe PLMN identifiers, Tracking Area Code (TAC), and cell ID of anetwork, a cell barring state indicative of whether a cell is a cell onwhich UE can camp, a required minimum reception level within a cellwhich is used as a cell reselection reference, and the transmission timeand period of other SIBs.

The SIB2 may include radio resource configuration information common toall types of UE. The SIB2 may include information related to an uplinkcarrier frequency and uplink channel bandwidth, an RACH configuration, apage configuration, an uplink power control configuration, a soundingreference signal configuration, a PUCCH configuration supportingACK/NACK transmission, and a PUSCH configuration.

UE may apply a procedure for obtaining system information and fordetecting a change of system information to only a PCell. In an SCell,when the corresponding SCell is added, the E-UTRAN may provide all typesof system information related to an RRC connection state operationthrough dedicated signaling. When system information related to aconfigured SCell is changed, the E-UTRAN may release a considered SCelland add the considered SCell later. This may be performed along with asingle RRC connection re-establishment message. The E-UTRAN may set avalue broadcast within a considered SCell and other parameter valuethrough dedicated signaling.

UE needs to guarantee the validity of a specific type of systeminformation. Such system information is called required systeminformation. The required system information may be defined as follows.

If UE is in the RRC_IDLE state: the UE needs to have the valid versionof the MIB and the SIB1 in addition to the SIB2 to SIB8. This may complywith the support of a considered RAT.

If UE is in the RRC connection state: the UE needs to have the validversion of the MIB, SIB1, and SIB2.

In general, the validity of system information may be guaranteed up to amaximum of 3 hours after being obtained.

In general, service that is provided to UE by a network may beclassified into three types as follows. Furthermore, the UE differentlyrecognizes the type of cell depending on what service may be provided tothe UE. In the following description, a service type is first described,and the type of cell is described.

1) Limited service: this service provides emergency calls and anEarthquake and Tsunami Warning System (ETWS), and may be provided by anacceptable cell.

2) Suitable service: this service means public service for common uses,and may be provided by a suitable cell (or a normal cell).

3) Operator service: this service means service for communicationnetwork operators. This cell may be used by only communication networkoperators, but may not be used by common users.

In relation to a service type provided by a cell, the type of cell maybe classified as follows.

1) An acceptable cell: this cell is a cell from which UE may be providedwith limited service. This cell is a cell that has not been barred froma viewpoint of corresponding UE and that satisfies the cell selectioncriterion of the UE.

2) A suitable cell: this cell is a cell from which UE may be providedwith suitable service. This cell satisfies the conditions of anacceptable cell and also satisfies additional conditions. The additionalconditions include that the suitable cell needs to belong to a PublicLand Mobile Network (PLMN) to which corresponding UE may access and thatthe suitable cell is a cell on which the execution of a tracking areaupdate procedure by the UE is not barred. If a corresponding cell is aCSG cell, the cell needs to be a cell to which UE may access as a memberof the CSG.

3) A barred cell: this cell is a cell that broadcasts informationindicative of a barred cell through system information.

4) A reserved cell: this cell is a cell that broadcasts informationindicative of a reserved cell through system information.

FIG. 4 is a flowchart illustrating the operation of UE in the RRC idlestate. FIG. 4 illustrates a procedure in which UE that is initiallypowered on experiences a cell selection process, registers it with anetwork, and then performs cell reselection if necessary.

Referring to FIG. 4, the UE selects Radio Access Technology (RAT) inwhich the UE communicates with a Public Land Mobile Network (PLMN), thatis, a network from which the UE is provided with service (S410).Information about the PLMN and the RAT may be selected by the user ofthe UE, and the information stored in a Universal Subscriber IdentityModule (USIM) may be used.

The UE selects a cell that has the greatest value and that belongs tocells having measured BS and signal intensity or quality greater than aspecific value (cell selection) (S420). In this case, the UE that ispowered off performs cell selection, which may be called initial cellselection. A cell selection procedure is described later in detail.After the cell selection, the UE receives system informationperiodically by the BS. The specific value refers to a value that isdefined in a system in order for the quality of a physical signal indata transmission/reception to be guaranteed. Accordingly, the specificvalue may differ depending on applied RAT.

If network registration is necessary, the UE performs a networkregistration procedure (S430). The UE registers its information (e.g.,an IMSI) with the network in order to receive service (e.g., paging)from the network. The UE does not register it with a network whenever itselects a cell, but registers it with a network when information aboutthe network (e.g., a Tracking Area Identity (TAI)) included in systeminformation is different from information about the network that isknown to the UE.

The UE performs cell reselection based on a service environment providedby the cell or the environment of the UE (S440). If the value of theintensity or quality of a signal measured based on a BS from which theUE is provided with service is lower than that measured based on a BS ofa neighboring cell, the UE selects a cell that belongs to other cellsand that provides better signal characteristics than the cell of the BSthat is accessed by the UE. This process is called cell reselectiondifferently from the initial cell selection of the No. 2 process. Inthis case, temporal restriction conditions are placed in order for acell to be frequently reselected in response to a change of signalcharacteristic. A cell reselection procedure is described later indetail.

FIG. 5 is a flowchart illustrating a process of establishing RRCconnection.

UE sends an RRC connection request message that requests RRC connectionto a network (S510). The network sends an RRC connection establishmentmessage as a response to the RRC connection request (S520). Afterreceiving the RRC connection establishment message, the UE enters RRCconnected mode.

The UE sends an RRC connection establishment complete message used tocheck the successful completion of the RRC connection to the network(S530).

FIG. 6 is a flowchart illustrating an RRC connection reconfigurationprocess. An RRC connection reconfiguration is used to modify RRCconnection. This is used to establish/modify/release RBs, performhandover, and set up/modify/release measurements.

A network sends an RRC connection reconfiguration message for modifyingRRC connection to UE (S610). As a response to the RRC connectionreconfiguration message, the UE sends an RRC connection reconfigurationcomplete message used to check the successful completion of the RRCconnection reconfiguration to the network (S620).

Hereinafter, a public land mobile network (PLMN) is described.

The PLMN is a network which is disposed and operated by a mobile networkoperator. Each mobile network operator operates one or more PLMNs. EachPLMN may be identified by a Mobile Country Code (MCC) and a MobileNetwork Code (MNC). PLMN information of a cell is included in systeminformation and broadcasted.

In PLMN selection, cell selection, and cell reselection, various typesof PLMNs may be considered by the terminal.

Home PLMN (HPLMN): PLMN having MCC and MNC matching with MCC and MNC ofa terminal IMSI.

Equivalent HPLMN (EHPLMN): PLMN serving as an equivalent of an HPLMN.

Registered PLMN (RPLMN): PLMN successfully finishing locationregistration.

Equivalent PLMN (EPLMN): PLMN serving as an equivalent of an RPLMN.

Each mobile service consumer subscribes in the HPLMN. When a generalservice is provided to the terminal through the HPLMN or the EHPLMN, theterminal is not in a roaming state. Meanwhile, when the service isprovided to the terminal through a PLMN except for the HPLMN/EHPLMN, theterminal is in the roaming state. In this case, the PLMN refers to aVisited PLMN (VPLMN).

When UE is initially powered on, the UE searches for available PublicLand Mobile Networks (PLMNs) and selects a proper PLMN from which the UEis able to be provided with service. The PLMN is a network that isdeployed or operated by a mobile network operator. Each mobile networkoperator operates one or more PLMNs. Each PLMN may be identified byMobile Country Code (MCC) and Mobile Network Code (MNC).

Information about the PLMN of a cell is included in system informationand broadcasted. The UE attempts to register it with the selected PLMN.If registration is successful, the selected PLMN becomes a RegisteredPLMN (RPLMN). The network may signalize a PLMN list to the UE. In thiscase, PLMNs included in the PLMN list may be considered to be PLMNs,such as RPLMNs. The UE registered with the network needs to be able tobe always reachable by the network. If the UE is in the ECM-CONNECTEDstate (identically the RRC connection state), the network recognizesthat the UE is being provided with service. If the UE is in the ECM-IDLEstate (identically the RRC idle state), however, the situation of the UEis not valid in an eNB, but is stored in the MME. In such a case, onlythe MME is informed of the location of the UE in the ECM-IDLE statethrough the granularity of the list of Tracking Areas (TAs). A single TAis identified by a Tracking Area Identity (TAI) formed of the identifierof a PLMN to which the TA belongs and Tracking Area Code (TAC) thatuniquely expresses the TA within the PLMN.

Thereafter, the UE selects a cell that belongs to cells provided by theselected PLMN and that has signal quality and characteristics on whichthe UE is able to be provided with proper service.

The following is a detailed description of a procedure of selecting acell by a terminal.

When power is turned-on or the terminal is located in a cell, theterminal performs procedures for receiving a service byselecting/reselecting a suitable quality cell.

A terminal in an RRC idle state should prepare to receive a servicethrough the cell by always selecting a suitable quality cell. Forexample, a terminal where power is turned-on just before should selectthe suitable quality cell to be registered in a network. If the terminalin an RRC connection state enters in an RRC idle state, the terminalshould selects a cell for stay in the RRC idle state. In this way, aprocedure of selecting a cell satisfying a certain condition by theterminal in order to be in a service idle state such as the RRC idlestate refers to cell selection. Since the cell selection is performed ina state that a cell in the RRC idle state is not currently determined,it is important to select the cell as rapid as possible. Accordingly, ifthe cell provides a wireless signal quality of a predetermined level orgreater, although the cell does not provide the best wireless signalquality, the cell may be selected during a cell selection procedure ofthe terminal.

A method and a procedure of selecting a cell by a terminal in a 3GPP LTEis described with reference to 3GPP TS 36.304 V8.5.0 (2009 March) “UserEquipment (UE) procedures in idle mode (Release 8)”.

A cell selection process is basically divided into two types.

The first is an initial cell selection process. In this process, UE doesnot have preliminary information about a wireless channel. Accordingly,the UE searches for all wireless channels in order to find out a propercell. The UE searches for the strongest cell in each channel.Thereafter, if the UE has only to search for a suitable cell thatsatisfies a cell selection criterion, the UE selects the correspondingcell.

Next, the UE may select the cell using stored information or usinginformation broadcasted by the cell. Accordingly, cell selection may befast compared to an initial cell selection process. If the UE has onlyto search for a cell that satisfies the cell selection criterion, the UEselects the corresponding cell. If a suitable cell that satisfies thecell selection criterion is not retrieved though such a process, the UEperforms an initial cell selection process.

The cell selection criterion may be defined as below equation 1.Srxlev>0 AND Squal>0  [Equation 1]where:Srxlev=Q _(rxlevmeas)−(Q _(rxlevmin) +Q _(rxlevminoffset))−P_(compensation)Squal=Q _(qualmeas)−(Q _(qualmin) +Q _(qualminoffset))

Here, the variables in the equation 1 may be defined as below table 1.

TABLE 1 Srxlev Cell selection RX level value (dB) Squal Cell selectionquality value (dB) Q_(rxlevmeas) Measured cell RX level value (RSRP)Q_(qualmeas) Measured cell quality value (RSRQ) Q_(rxlevmin) Minimumrequired RX level in the cell (dBm) Q_(qualmin) Minimum required qualitylevel in the cell (dB) Q_(rxlevminoffset) Offset to the signalledQ_(rxlevmin) taken into account in the Srxlev evaluation as a result ofa periodic search for a higher priority PLMN while camped normally in aVPLMN Q_(qualminoffset) Offset to the signalled Q_(qualmin) taken intoaccount in the Squal evaluation as a result of a periodic search for ahigher priority PLMN while camped normally in a VPLMN Pcompensationmax(P_(EMAX) − P_(PowerClass), 0) (dB) P_(EMAX) Maximum TX power levelan UE may use when transmitting on the uplink in the cell (dBm) definedas P_(EMAX) in [TS 36.101] P_(PowerClass) Maximum RF output power of theUE (dBm) according to the UE power class as defined in [TS 36.101]

Signalled values, i.e., Q_(rxlevminoffset) and Q_(qualminoffset), may beapplied to a case where cell selection is evaluated as a result ofperiodic search for a higher priority PLMN during a UE camps on a normalcell in a VPLMN. During the periodic search for the higher priority PLMNas described above, the UE may perform the cell selection evaluation byusing parameter values stored in other cells of the higher priorityPLMN.

After the UE selects a specific cell through the cell selection process,the intensity or quality of a signal between the UE and a BS may bechanged due to a change in the mobility or wireless environment of theUE. Accordingly, if the quality of the selected cell is deteriorated,the UE may select another cell that provides better quality. If a cellis reselected as described above, the UE selects a cell that providesbetter signal quality than the currently selected cell. Such a processis called cell reselection. In general, a basic object of the cellreselection process is to select a cell that provides UE with the bestquality from a viewpoint of the quality of a radio signal.

In addition to the viewpoint of the quality of a radio signal, a networkmay determine priority corresponding to each frequency, and may informthe UE of the determined priorities. The UE that has received thepriorities preferentially takes into consideration the priorities in acell reselection process compared to a radio signal quality criterion.

As described above, there is a method of selecting or reselecting a cellaccording to the signal characteristics of a wireless environment. Inselecting a cell for reselection when a cell is reselected, thefollowing cell reselection methods may be present according to the RATand frequency characteristics of the cell.

Intra-frequency cell reselection: UE reselects a cell having the samecenter frequency as that of RAT, such as a cell on which the UE campson.

Inter-frequency cell reselection: UE reselects a cell having a differentcenter frequency from that of RAT, such as a cell on which the UE campson

Inter-RAT cell reselection: UE reselects a cell that uses RAT differentfrom RAT on which the UE camps

The principle of a cell reselection process is as follows.

First, UE measures the quality of a serving cell and neighbor cells forcell reselection.

Second, cell reselection is performed based on a cell reselectioncriterion. The cell reselection criterion has the followingcharacteristics in relation to the measurements of a serving cell andneighbor cells.

Intra-frequency cell reselection is basically based on ranking. Rankingis a task for defining a criterion value for evaluating cell reselectionand numbering cells using criterion values according to the size of thecriterion values. A cell having the best criterion is commonly calledthe best-ranked cell. The cell criterion value is based on the value ofa corresponding cell measured by UE, and may be a value to which afrequency offset or cell offset has been applied, if necessary.

Inter-frequency cell reselection is based on frequency priority providedby a network. UE attempts to camp on a frequency having the highestfrequency priority. A network may provide frequency priority that willbe applied by UEs within a cell in common through broadcastingsignaling, or may provide frequency-specific priority to each UE throughUE-dedicated signaling. A cell reselection priority provided throughbroadcast signaling may refer to a common priority. A cell reselectionpriority for each terminal set by a network may refer to a dedicatedpriority. If receiving the dedicated priority, the terminal may receivea valid time associated with the dedicated priority together. Ifreceiving the dedicated priority, the terminal starts a validity timerset as the received valid time together therewith. While the valid timeris operated, the terminal applies the dedicated priority in the RRC idlemode. If the valid timer is expired, the terminal discards the dedicatedpriority and again applies the common priority.

For the inter-frequency cell reselection, a network may provide UE witha parameter (e.g., a frequency-specific offset) used in cell reselectionfor each frequency.

For the intra-frequency cell reselection or the inter-frequency cellreselection, a network may provide UE with a Neighboring Cell List (NCL)used in cell reselection. The NCL includes a cell-specific parameter(e.g., a cell-specific offset) used in cell reselection.

For the intra-frequency or inter-frequency cell reselection, a networkmay provide UE with a cell reselection black list used in cellreselection. The UE does not perform cell reselection on a cell includedin the black list.

Ranking performed in a cell reselection evaluation process is describedbelow.

A ranking criterion used to apply priority to a cell is defined as inEquation 1.Rs=Qmeas,s+Qhyst,Rn=Qmeas,s−Qoffset  [Equation 2]

In this case, Rs is the ranking criterion of a serving cell, Rn is theranking criterion of a neighbor cell, Qmeas,s is the quality value ofthe serving cell measured by UE, Qmeas,n is the quality value of theneighbor cell measured by UE, Qhyst is the hysteresis value for ranking,and Qoffset is an offset between the two cells.

In Intra-frequency, if UE receives an offset “Qoffsets,n” between aserving cell and a neighbor cell, Qoffset=Qoffsets,n. If UE does notQoffsets,n, Qoffset=0.

In Inter-frequency, if UE receives an offset “Qoffsets,n” for acorresponding cell, Qoffset=Qoffsets,n+Qfrequency. If UE does notreceive “Qoffsets,n”, Qoffset=Qfrequency.

If the ranking criterion Rs of a serving cell and the ranking criterionRn of a neighbor cell are changed in a similar state, ranking priorityis frequency changed as a result of the change, and UE may alternatelyreselect the twos. Qhyst is a parameter that gives hysteresis to cellreselection so that UE is prevented from to alternately reselecting twocells.

UE measures RS of a serving cell and Rn of a neighbor cell according tothe above equation, considers a cell having the greatest rankingcriterion value to be the best-ranked cell, and reselects the cell.

In accordance with the criterion, it may be checked that the quality ofa cell is the most important criterion in cell reselection. If areselected cell is not a suitable cell, UE excludes a correspondingfrequency or a corresponding cell from the subject of cell reselection.

A Radio Link Failure (RLF) is described below.

UE continues to perform measurements in order to maintain the quality ofa radio link with a serving cell from which the UE receives service. TheUE determines whether or not communication is impossible in a currentsituation due to the deterioration of the quality of the radio link withthe serving cell. If communication is almost impossible because thequality of the serving cell is too low, the UE determines the currentsituation to be an RLF.

If the RLF is determined, the UE abandons maintaining communication withthe current serving cell, selects a new cell through cell selection (orcell reselection) procedure, and attempts RRC connectionre-establishment with the new cell.

In the specification of 3GPP LTE, the following examples are taken ascases where normal communication is impossible.

A case where UE determines that there is a serious problem in thequality of a downlink communication link (a case where the quality of aPCell is determined to be low while performing RLM) based on the radioquality measured results of the PHY layer of the UE

A case where uplink transmission is problematic because a random accessprocedure continues to fail in the MAC sublayer.

A case where uplink transmission is problematic because uplink datatransmission continues to fail in the RLC sublayer.

A case where handover is determined to have failed.

A case where a message received by UE does not pass through an integritycheck.

An RRC connection re-establishment procedure is described in more detailbelow.

FIG. 7 is a diagram illustrating an RRC connection re-establishmentprocedure.

Referring to FIG. 7, UE stops using all the radio bearers that have beenconfigured other than a Signaling Radio Bearer (SRB) #0, and initializesa variety of kinds of sublayers of an Access Stratum (AS) (S710).Furthermore, the UE configures each sublayer and the PHY layer as adefault configuration. In this process, the UE maintains the RRCconnection state.

The UE performs a cell selection procedure for performing an RRCconnection reconfiguration procedure (S720). The cell selectionprocedure of the RRC connection re-establishment procedure may beperformed in the same manner as the cell selection procedure that isperformed by the UE in the RRC idle state, although the UE maintains theRRC connection state.

After performing the cell selection procedure, the UE determines whetheror not a corresponding cell is a suitable cell by checking the systeminformation of the corresponding cell (S730). If the selected cell isdetermined to be a suitable E-UTRAN cell, the UE sends an RRC connectionre-establishment request message to the corresponding cell (S740).

Meanwhile, if the selected cell is determined to be a cell that uses RATdifferent from that of the E-UTRAN through the cell selection procedurefor performing the RRC connection re-establishment procedure, the UEstops the RRC connection re-establishment procedure and enters the RRCidle state (S750).

The UE may be implemented to finish checking whether the selected cellis a suitable cell through the cell selection procedure and thereception of the system information of the selected cell. To this end,the UE may drive a timer when the RRC connection re-establishmentprocedure is started. The timer may be stopped if it is determined thatthe UE has selected a suitable cell. If the timer expires, the UE mayconsider that the RRC connection re-establishment procedure has failed,and may enter the RRC idle state. Such a timer is hereinafter called anRLF timer. In LTE spec TS 36.331, a timer named “T311” may be used as anRLF timer. The UE may obtain the set value of the timer from the systeminformation of the serving cell.

If an RRC connection re-establishment request message is received fromthe UE and the request is accepted, a cell sends an RRC connectionre-establishment message to the UE.

The UE that has received the RRC connection re-establishment messagefrom the cell reconfigures a PDCP sublayer and an RLC sublayer with anSRB1. Furthermore, the UE calculates various key values related tosecurity setting, and reconfigures a PDCP sublayer responsible forsecurity as the newly calculated security key values. Accordingly, theSRB 1 between the UE and the cell is open, and the UE and the cell mayexchange RRC control messages. The UE completes the restart of the SRB1,and sends an RRC connection re-establishment complete message indicativeof that the RRC connection re-establishment procedure has been completedto the cell (S760).

In contrast, if the RRC connection re-establishment request message isreceived from the UE and the request is not accepted, the cell sends anRRC connection re-establishment reject message to the UE.

If the RRC connection re-establishment procedure is successfullyperformed, the cell and the UE perform an RRC connection reconfigurationprocedure. Accordingly, the UE recovers the state prior to the executionof the RRC connection re-establishment procedure, and the continuity ofservice is guaranteed to the upmost.

FIG. 8 illustrates substates which may be owned by UE in the RRC_IDLEstate and a substate transition process.

Referring to FIG. 8, UE performs an initial cell selection process(S801). The initial cell selection process may be performed when thereis no cell information stored with respect to a PLMN or if a suitablecell is not discovered.

If a suitable cell is unable to be discovered in the initial cellselection process, the UE transits to any cell selection state (S802).The any cell selection state is the state in which the UE has not campedon a suitable cell and an acceptable cell and is the state in which theUE attempts to discover an acceptable cell of a specific PLMN on whichthe UE may camp. If the UE has not discovered any cell on which it maycamp, the UE continues to stay in the any cell selection state until itdiscovers an acceptable cell.

If a suitable cell is discovered in the initial cell selection process,the UE transits to a normal camp state (S803). The normal camp staterefers to the state in which the UE has camped on the suitable cell. Inthis state, the UE may select and monitor a paging channel based oninformation provided through system information and may perform anevaluation process for cell reselection.

If a cell reselection evaluation process (S804) is caused in the normalcamp state (S803), the UE performs a cell reselection evaluation process(S804). If a suitable cell is discovered in the cell reselectionevaluation process (S804), the UE transits to the normal camp state(S803) again.

If an acceptable cell is discovered in the any cell selection state(S802), the UE transmits to any cell camp state (S805). The any cellcamp state is the state in which the UE has camped on the acceptablecell.

In the any cell camp state (S805), the UE may select and monitor apaging channel based on information provided through system informationand may perform the evaluation process (S806) for cell reselection. Ifan acceptable cell is not discovered in the evaluation process (S806)for cell reselection, the UE transits to the any cell selection state(S802).

Now, a device-to-device (D2D) operation is described. In 3GPP LTE-A, aservice related to the D2D operation is called a proximity service(ProSe). Now, the ProSe is described. Hereinafter, the ProSe is the sameconcept as the D2D operation, and the ProSe and the D2D operation may beused without distinction.

The ProSe includes ProSe direction communication and ProSe directdiscovery. The ProSe direct communication is communication performedbetween two or more proximate UEs. The UEs may perform communication byusing a protocol of a user plane. A ProSe-enabled UE implies a UEsupporting a procedure related to a requirement of the ProSe. Unlessotherwise specified, the ProSe-enabled UE includes both of a publicsafety UE and a non-public safety UE. The public safety UE is a UEsupporting both of a function specified for a public safety and a ProSeprocedure, and the non-public safety UE is a UE supporting the ProSeprocedure and not supporting the function specified for the publicsafety.

ProSe direct discovery is a process for discovering anotherProSe-enabled UE adjacent to ProSe-enabled UE. In this case, only thecapabilities of the two types of ProSe-enabled UE are used. EPC-levelProSe discovery means a process for determining, by an EPC, whether thetwo types of ProSe-enabled UE are in proximity and notifying the twotypes of ProSe-enabled UE of the proximity.

Hereinafter, for convenience, the ProSe direct communication may bereferred to as D2D communication, and the ProSe direct discovery may bereferred to as D2D discovery.

FIG. 9 shows a basic structure for ProSe.

Referring to FIG. 9, the basic structure for ProSe includes an E-UTRAN,an EPC, a plurality of types of UE including a ProSe applicationprogram, a ProSe application server (a ProSe APP server), and a ProSefunction.

The EPC represents an E-UTRAN core network configuration. The EPC mayinclude an MME, an S-GW, a P-GW, a policy and charging rules function(PCRF), a home subscriber server (HSS) and so on.

The ProSe APP server is a user of a ProSe capability for producing anapplication function. The ProSe APP server may communicate with anapplication program within UE. The application program within UE may usea ProSe capability for producing an application function.

The ProSe function may include at least one of the followings, but isnot necessarily limited thereto.

Interworking via a reference point toward the 3rd party applications

Authorization and configuration of UE for discovery and directcommunication

Enable the functionality of EPC level ProSe discovery

ProSe related new subscriber data and handling of data storage, and alsohandling of the ProSe identities

Security related functionality

Provide control towards the EPC for policy related functionality

Provide functionality for charging (via or outside of the EPC, e.g.,offline charging)

A reference point and a reference interface in the basic structure forProSe are described below.

PC1: a reference point between the ProSe application program within theUE and the ProSe application program within the ProSe APP server. Thisis used to define signaling requirements in an application dimension.

PC2: a reference point between the ProSe APP server and the ProSefunction. This is used to define an interaction between the ProSe APPserver and the ProSe function. The update of application data in theProSe database of the ProSe function may be an example of theinteraction.

PC3: a reference point between the UE and the ProSe function. This isused to define an interaction between the UE and the ProSe function. Aconfiguration for ProSe discovery and communication may be an example ofthe interaction.

PC4: a reference point between the EPC and the ProSe function. This isused to define an interaction between the EPC and the ProSe function.The interaction may illustrate the time when a path for 1:1communication between types of UE is set up or the time when ProSeservice for real-time session management or mobility management isauthenticated.

PC5: a reference point used for using control/user plane for discoveryand communication, relay, and 1:1 communication between types of UE.

PC6: a reference point for using a function, such as ProSe discovery,between users belonging to different PLMNs.

SGi: this may be used to exchange application data and types ofapplication dimension control information.

<ProSe Direct Communication>

ProSe direct communication is communication mode in which two types ofpublic safety UE can perform direct communication through a PC 5interface. Such communication mode may be supported when UE is suppliedwith services within coverage of an E-UTRAN or when UE deviates fromcoverage of an E-UTRAN.

FIG. 10 shows the deployment examples of types of UE performing ProSedirect communication and cell coverage.

Referring to FIG. 10(a), types of UE A and B may be placed outside cellcoverage. Referring to FIG. 10(b), UE A may be placed within cellcoverage, and UE B may be placed outside cell coverage. Referring toFIG. 10(c), types of UE A and B may be placed within single cellcoverage. Referring to FIG. 10(d), UE A may be placed within coverage ofa first cell, and UE B may be placed within coverage of a second cell.

ProSe direct communication may be performed between types of UE placedat various positions as in FIG. 10.

Meanwhile, the following IDs may be used in ProSe direct communication.

A source layer-2 ID: this ID identifies the sender of a packet in the PC5 interface.

A destination layer-2 ID: this ID identifies the target of a packet inthe PC 5 interface.

An SA L1 ID: this ID is the ID of scheduling assignment (SA) in the PC 5interface.

FIG. 11 shows a user plane protocol stack for ProSe directcommunication.

Referring to FIG. 11, the PC 5 interface includes a PDCH, RLC, MAC, andPHY layers.

In ProSe direct communication, HARQ feedback may not be present. An MACheader may include a source layer-2 ID and a destination layer-2 ID.

<Radio Resource Assignment for ProSe Direct Communication>

ProSe-enabled UE may use the following two types of mode for resourceassignment for ProSe direct communication.

1. Mode 1

Mode 1 is mode in which resources for ProSe direct communication arescheduled by an eNB. UE needs to be in the RRC_CONNECTED state in orderto send data in accordance with mode 1. The UE requests a transmissionresource from an eNB. The eNB performs scheduling assignment andschedules resources for sending data. The UE may send a schedulingrequest to the eNB and send a ProSe Buffer Status Report (BSR). The eNBhas data to be subjected to ProSe direct communication by the UE basedon the ProSe BSR and determines that a resource for transmission isrequired.

2. Mode 2

Mode 2 is mode in which UE directly selects a resource. UE directlyselects a resource for ProSe direct communication in a resource pool.The resource pool may be configured by a network or may have beenpreviously determined.

Meanwhile, if UE has a serving cell, that is, if the UE is in theRRC_CONNECTED state with an eNB or is placed in a specific cell in theRRC_IDLE state, the UE is considered to be placed within coverage of theeNB.

If UE is placed outside coverage, only mode 2 may be applied. If the UEis placed within the coverage, the UE may use mode 1 or mode 2 dependingon the configuration of an eNB.

If another exception condition is not present, only when an eNB performsa configuration, UE may change mode from mode 1 to mode 2 or from mode 2to mode 1.

<ProSe Direct Discovery>

ProSe direct discovery refers to a procedure that is used forProSe-enabled UE to discover another ProSe-enabled UE in proximity andis also called D2D direct discovery. In this case, E-UTRA radio signalsthrough the PC 5 interface may be used. Information used in ProSe directdiscovery is hereinafter called discovery information.

FIG. 12 shows the PC 5 interface for D2D direct discovery.

Referring to FIG. 12, the PC 5 interface includes an MAC layer, a PHYlayer, and a ProSe Protocol layer, that is, a higher layer. The higherlayer (the ProSe Protocol) handles the permission of the announcementand monitoring of discovery information. The contents of the discoveryinformation are transparent to an access stratum (AS). The ProSeProtocol transfers only valid discovery information to the AS forannouncement.

The MAC layer receives discovery information from the higher layer (theProSe Protocol). An IP layer is not used to send discovery information.The MAC layer determines a resource used to announce discoveryinformation received from the higher layer. The MAC layer produces anMAC protocol data unit (PDU) for carrying discovery information andsends the MAC PDU to the physical layer. An MAC header is not added.

In order to announce discovery information, there are two types ofresource assignment.

1. Type 1

The type 1 is a method for assigning a resource for announcing discoveryinformation in a UE-not-specific manner. An eNB provides a resource poolconfiguration for discovery information announcement to types of UE. Theconfiguration may be signaled through the SIB.

UE autonomously selects a resource from an indicated resource pool andannounces discovery information using the selected resource. The UE mayannounce the discovery information through a randomly selected resourceduring each discovery period.

2. Type 2

The type 2 is a method for assigning a resource for announcing discoveryinformation in a UE-specific manner. UE in the RRC_CONNECTED state mayrequest a resource for discovery signal announcement from an eNB throughan RRC signal. The eNB may announce a resource for discovery signalannouncement through an RRC signal. A resource for discovery signalmonitoring may be assigned within a resource pool configured for typesof UE.

An eNB 1) may announce a type 1 resource pool for discovery signalannouncement to UE in the RRC_IDLE state through the SIB. Types of UEwhose ProSe direct discovery has been permitted use the type 1 resourcepool for discovery information announcement in the RRC_IDLE state.Alternatively, the eNB 2) announces that the eNB supports ProSe directdiscovery through the SIB, but may not provide a resource for discoveryinformation announcement. In this case, UE needs to enter theRRC_CONNECTED state for discovery information announcement.

An eNB may configure that UE has to use a type 1 resource pool fordiscovery information announcement or has to use a type 2 resourcethrough an RRC signal in relation to UE in the RRC_CONNECTED state.

FIG. 13 is an embodiment of a ProSe discovery process.

Referring to FIG. 13, it is assumed that UE A and UE B haveProSe-enabled application programs managed therein and have beenconfigured to have a ‘friend’ relation between them in the applicationprograms, that is, a relationship in which D2D communication may bepermitted between them. Hereinafter, the UE B may be represented as a‘friend’ of the UE A. The application program may be, for example, asocial networking program. ‘3GPP Layers’ correspond to the functions ofan application program for using ProSe discovery service, which havebeen defined by 3GPP.

Direct discovery between the types of UE A and B may experience thefollowing process.

1. First, the UE A performs regular application layer communication withthe APP server. The communication is based on an Application ProgramInterface (API).

2. The ProSe-enabled application program of the UE A receives a list ofapplication layer IDs having a ‘friend’ relation. In general, theapplication layer ID may have a network access ID form. For example, theapplication layer ID of the UE A may have a form, such as“adam@example.com.”

3. The UE A requests private expressions code for the user of the UE Aand private representation code for a friend of the user.

4. The 3GPP layers send a representation code request to the ProSeserver.

5. The ProSe server maps the application layer IDs, provided by anoperator or a third party APP server, to the private representationcode. For example, an application layer ID, such as adam@example.com,may be mapped to private representation code, such as“GTER543$#2FSJ67DFSF.” Such mapping may be performed based on parameters(e.g., a mapping algorithm, a key value and so on) received from the APPserver of a network.

6. The ProSe server sends the types of derived representation code tothe 3GPP layers. The 3GPP layers announce the successful reception ofthe types of representation code for the requested application layer IDto the ProSe-enabled application program. Furthermore, the 3GPP layersgenerate a mapping table between the application layer ID and the typesof representation code.

7. The ProSe-enabled application program requests the 3GPP layers tostart a discovery procedure. That is, the ProSe-enabled applicationprogram requests the 3GPP layers to start discovery when one of provided‘friends’ is placed in proximity to the UE A and direct communication ispossible. The 3GPP layers announces the private representation code(i.e., in the above example, “GTER543$#2FSJ67DFSF”, that is, the privaterepresentation code of adam@example.com) of the UE A. This ishereinafter called ‘announcement’. Mapping between the application layerID of the corresponding application program and the privaterepresentation code may be known to only ‘friends’ which have previouslyreceived such a mapping relation, and the ‘friends’ may perform suchmapping.

8. It is assumed that the UE B operates the same ProSe-enabledapplication program as the UE A and has executed the aforementioned 3 to6 steps. The 3GPP layers placed in the UE B may execute ProSe discovery.

9. When the UE B receives the aforementioned ‘announce’ from the UE A,the UE B determines whether the private representation code included inthe ‘announce’ is known to the UE B and whether the privaterepresentation code is mapped to the application layer ID. As describedthe 8 step, since the UE B has also executed the 3 to 6 steps, it isaware of the private representation code, mapping between the privaterepresentation code and the application layer ID, and correspondingapplication program of the UE A. Accordingly, the UE B may discover theUE A from the ‘announce’ of the UE A. The 3GPP layers announce thatadam@example.com has been discovered to the ProSe-enabled applicationprogram within the UE B.

In FIG. 13, the discovery procedure has been described by taking intoconsideration all of the types of UE A and B, the ProSe server, the APPserver and so on. From the viewpoint of the operation between the typesof UE A and B, the UE A sends (this process may be called announcement)a signal called announcement, and the UE B receives the announce anddiscovers the UE A. That is, from the aspect that an operation thatbelongs to operations performed by types of UE and that is directlyrelated to another UE is only step, the discovery process of FIG. 13 mayalso be called a single step discovery procedure.

FIG. 14 is another embodiment of a ProSe discovery process.

In FIG. 14, types of UE 1 to 4 are assumed to types of UE included inspecific group communication system enablers (GCSE) group. It is assumedthat the UE 1 is a discoverer and the types of UE 2, 3, and 4 arediscoveree. UE 5 is UE not related to the discovery process.

The UE 1 and the UE 2-4 may perform a next operation in the discoveryprocess.

First, the UE 1 broadcasts a target discovery request message (may behereinafter abbreviated as a discovery request message or M1) in orderto discover whether specific UE included in the GCSE group is inproximity. The target discovery request message may include the uniqueapplication program group ID or layer-2 group ID of the specific GCSEgroup. Furthermore, the target discovery request message may include theunique ID, that is, application program private ID of the UE 1. Thetarget discovery request message may be received by the types of UE 2,3, 4, and 5.

The UE 5 sends no response message. In contrast, the types of UE 2, 3,and 4 included in the GCSE group send a target discovery responsemessage (may be hereinafter abbreviated as a discovery response messageor M2) as a response to the target discovery request message. The targetdiscovery response message may include the unique application programprivate ID of UE sending the message.

An operation between types of UE in the ProSe discovery processdescribed with reference to FIG. 14 is described below. The discoverer(the UE 1) sends a target discovery request message and receives atarget discovery response message, that is, a response to the targetdiscovery request message. Furthermore, when the discoveree (e.g., theUE 2) receives the target discovery request message, it sends a targetdiscovery response message, that is, a response to the target discoveryrequest message. Accordingly, each of the types of UE performs theoperation of the 2 step. In this aspect, the ProSe discovery process ofFIG. 14 may be called a 2-step discovery procedure.

In addition to the discovery procedure described in FIG. 14, if the UE 1(the discoverer) sends a discovery conform message (may be hereinafterabbreviated as an M3), that is, a response to the target discoveryresponse message, this may be called a 3-step discovery procedure.

<Synchronization Signal in D2D Operation>

According to the related art, a synchronization signal is transmitted bya center network node (e.g., base station) using a downlink resource.However, in the D2D operation, the synchronization signal may betransmitted by a terminal. In particular, when the synchronizationsignal transmitted through a base station is not detected or is too weakto be identified, a synchronization signal may be transmitted by aterminal for the D2D operation between terminals.

That is, an exact reception/decoding of a wireless signal may bepossible by adjusting synchronization between terminals performing theD2D operation. The synchronization signal is used to acquiresynchronization of a time and a frequency. A network node which is not abase station, for example, a terminal may transmit the synchronizationsignal in the D2D operation. Hereinafter, unless other defined, thesynchronization signal means a synchronization signal in the D2Doperation, that is, a synchronization signal transmitted from a networknode which is not a base station. Further, hereinafter, thesynchronization signal may mean a signal having all or a part offollowing characteristics.

1) The synchronization signal is regarded to be transmitted by theterminal. 2) If a second terminal receiving a synchronization signaltransmitted from the first terminal performs synchronization based onthe synchronization signal, the second terminal may adjustsynchronization for receiving a D2D signal transmitted from the firstterminal and a D2D signal transmitted from a third terminal which isanother terminal synchronized based on the synchronization signal. 3)The synchronization signal is transmitted through an uplink channel. 4)The synchronization signal is transmitted through uplink resource/uplinksubframe/uplink frequency.

When the terminal provides a specific sequence (refers tosynchronization sequence) which may be used as a reference ofsynchronization to another terminal, the terminal may broadcastinformation including an indicator indicating whether the terminal islocated in network coverage. A terminal receiving the specific sequencemay determine whether the specific sequence is a synchronization signalused within the network coverage or a synchronization signal usedoutside the network coverage.

Meanwhile, if a terminal receives the synchronization signal at time t1,the terminal may transmit a synchronization signal at time t1-t2. Inthis case, the t2 may have a positive value, a negative value or 0 asoffset. A value of the t2 may be regulated as a fixed value, may be setby a network or may be induced from PUSCH transmission timing of a cellto which the terminal belongs.

A synchronization signal used for a D2D operation transmitted from anetwork node (e.g., terminal) except for a base station may transfer anID of a subject transmitting the synchronization signal and/or a type ofthe subject.

The synchronization signal may include a primary synchronization signaland a secondary synchronization signal. The primary synchronizationsignal may use a Zadoff Chu sequence and the secondary synchronizationsignal may use an M sequence. The Zadoff Chu sequence includes constantamplitude and zero correlation. The M sequence is a type of apseudorandom binary sequence.

In a following description, uplink means communication from a terminalto a base station. The network node may represent a terminal, a basestation, or both of them. A configuration may signify a rule which isdetermined by the network or is previously determined in the terminal.

Meanwhile, the terminal for supporting the D2D operation may serve as arelay.

FIG. 15 illustrates a UE-NW relay.

Referring to FIG. 15, a UE 2 153 serves as an UE-NW relay. That is, theUE 2 153 is a network node configured to relay between a UE 1 152located outside coverage 154 of a network 151 and the network 151. Inthis case, the UE 2 153 may serve as an UE-NE relay. A D2D operation maybe performed between UE 1 152 and the UE 2 153, and existing cellularcommunication may be performed between the UE 2 153 and the network 151.In FIG. 15, since the UE 1 152 is located outside the network coverage154, if the UE 2 153 does not provide a relay function, the UE 2 153 maynot communicate with the network 151. The UE-NW relay transmits andreceives data to and from the UE 1 through D2D communication (D2Doperation), and transmits and receives the data to and from the networkthrough general UE-network communication.

FIG. 16 illustrates a UE-UE relay.

Referring to FIG. 16, a UE 2 163 serves as an UE-UE relay. That is, theUE 2 163 is a network node configured to relay between another UE 161located outside coverage of a specific UE 162 and the specific UE 162.In this case, the UE 2 163 may serve as an UE-UE relay. In FIG. 16, theUE 1 162 and the UE 3 161 are located outside coverage thereof to eachother, if the UE 2 163 does not provide the relay function, the UE 1 162may not communicate with the UE 3 161. The D2D operation may beperformed between the UE 1 162 and the UE 2 163, and between the UE 2163 and the UE 3 161. The UE-UE relay transmits and receives data to andfrom the UE 1 through D2D communication (D2D operation), and transmitsand receives the data to and from the UE 3 through the D2D communication(D2D operation).

Hereinafter, the present invention will be described. The presentinvention relates to a D2D operation method performed by a UE how toreceive and use a D2D resource during a D2D operation.

FIG. 17 illustrates a D2D operation method performed by a UE accordingto an embodiment of the present invention.

Referring to FIG. 17, a UE receives D2D configuration informationindicating a plurality of resources which may be used in a D2D operationfrom a network (S191). An individual resource configuring a plurality ofresources may be a resource pool. That is, the plurality of resourcesmay be a list of resource pools. D2D configuration information may beprovided through broadcasted system information, or may beUE-specifically provided, and may be provided through an RRC message.

The UE selects a specific resource from the plurality of resources(S192). In this case, the UE may select the specific resource based onreference signal received power (RSRP) received from the network.

The UE performs the D2D operation with another UE using the selectedspecific resource (S193). To select and use the specific resource isexceptional by taking into consideration transmission using only aresource strictly allowed by the network. The above is applicable to theD2D operation. That is, a plurality of resources may include a resourcerelated to D2D transmission.

Hereinafter, respective steps will be described in detail.

Following tables 2 to 5 illustrate detailed examples of D2Dconfiguration information indicating a plurality of resources which maybe used in the D2D operation.

TABLE 2 ProseCommConfig information element -- ASN1STARTProseCommConfig-r12 ::= SEQUENCE { commTxResources-r12 CHOICE { releaseNULL, setup CHOICE { scheduled-r12 SEQUENCE { s1-RNTI-r12 C-RNTI,bsr-Config-r12 ProseBSR-Config-r12, commTxConfig-r12ProseCommResourcePool-r12, mcs-r12 INTEGER (0..28) OPTIONAL -- Need OP}, ue-Selected-r12 SEQUENCE { -- Pool for normal usagecommTxPoolNormalDedicated-r12 SEQUENCE { poolToReleaseList-r12ProseTxPoolToReleaseList-r12 OPTIONAL, -- Need ON poolToAddModList-r12ProseCommTxPoolToAddModList-r12 OPTIONAL -- Need ON } } } } ...OPTIONAL, -- Need ON } ProseCommTxPoolToAddModList-r12 ::= SEQUENCE(SIZE (1..maxProseTxPool-r12)) OF ProseCommTxPoolToAddMod-r12ProseCommTxPoolToAddMod-r12 ::= SEQUENCE { poolIdentity-r12ProseTxPoolIdentity-r12, pool-r12 ProseCommResourcePool-r12 }ProseBSR-Config-r12 ::= SEQUENCE { periodicBSR-Timer ENUMERATED { sf5,sf10, sfl6, sf20, sf32, sf40, sf64, sf80, sf128, sf160, sf320, sf640,sf1280, sf2560, infinity, spare1}, retxBSR-Timer ENUMERATED { sf320,sf640, sf1280, sf2560, sf5120, sf10240, spare2, spare1} } -- ASN1STOP

In the table 2, the ‘ProseCommConfig’ defines dedicated configurationinformation for ProSe direct communication: D2D communication), and inparticular, is related to transmission resource configuration for D2Dcommunication.

In the table 2, the ‘ProseCommResourcePool’ may indicate a plurality ofresource pools for D2D communication, and may include configurationinformation for each resource pool. A following table 3 illustrates anexample of the ‘ProseCommResourcePool’.

TABLE 3 -- ASN1START ProseCommPoolList4-r12 ::= SEQUENCE (SIZE(1..maxProseTxPool-r12)) OF ProseComm Resource Pool-r12ProseCommPoolList16-r12 ::= SEQUENCE (SIZE (1..maxProseRxPool-r12)) OFProseComm Resource Pool-r12 ProseCommResourcePool-r12 ::= SEQUENCE {sc-CP-Len-r12 Prose-CP-Len-r12, sc-Period-r12 ENUMERATED {sf40, sf60,sf70, sf80, sf120, sf140,  sf160, sf20, sf260, sf280, sf320},sc-TF-ResourceConfig-r12 Prose-TF-ResourceConfig-r12, data-CP-Len-r12Prose-CP-Len-r12, dataHoppingConfig-r12 Prose-HoppingConfigComm-r12,ue-SelectedResourceConfig SEQUENCE { -- Parameters not used in case ofscheduled Tx config  data-TF-ResourceConfig Prose-TF-ResourceConfig-r12, trpt-Subset-r12 BIT STRING (SIZE (3..5)) OPTIONAL -- Need OR }OPTIONAL, -- Need OR rx-ParametersNCell SEQUENCE {  tdd-Config-r12TDD-Config OPTIONAL, -- Need OR  sync-ConfigIndex-r12 INTEGER (0..15) }OPTIONAL, -- Need OR tx-Parameters SEQUENCE {  sc-TxParameters-r12Prose-TxParameters-r12,  dataTxParameters-r12 Prose-TxParameters-r12 }OPTIONAL, -- Need OR ... } Prose-CP-Len-r12 ::= ENUMERATED {normal,extended} Prose-HoppingConfigComm-r12 ::= SEQUENCE { hoppingParameter-r12 INTEGER (0..504), numSubbands-r12 ENUMERATED {ns1, ns2,ns4}, rb-Offset-r12 INTEGER (0..110) } -- ASN1STOP

In the table 3, the ‘ProseCommPoolList4’ is a list which includes thenumber of ‘ProseCommResourcePool’ corresponding to the number of‘maxProseTxPool’, and defines resources with respect to signaltransmission related to the D2D communication. The ‘ProseCommPoolList16’is a list with the number of ‘ProseCommResourcePool’ corresponding tothe number of ‘maxProseRxPool’, and defined resources with respect tosignal reception related to the D2D communication.

TABLE 4 ProseDiscConfig information element -- ASN1STARTProseDiscConfig-r12 ::= SEQUENCE { discTxResources-r12 CHOICE { releaseNULL, setup CHOICE { scheduled-r12 SEQUENCE { discTxConfig-r12ProseDiscResourcePool-r12 OPTIONAL, -- Need ON discTF-IndexList-r12ProseTF-IndexPairList-r12 OPTIONAL, -- Need ON discHoppingConfig-r12ProseHoppingConfigDisc-r12 OPTIONAL - - Need OR }, ue-Selected-r12SEQUENCE { discTxPoolDedicated-r12 SEQUENCE { poolToReleaseList-r12ProseTxPoolToReleaseList-r12 OPTIONAL, - - Need ON poolToAddModList-r12ProseDiscTxPoolToAddModList-r12 OPTIONAL - - Need ON } OPTIONAL -- NeedON } } } OPTIONAL, -- Need ON ... } ProseDiscTxPoolToAddModList-r12 : := SEQUENCE (SIZE (1..maxProseTxPool-r12)) OF ProseDiscTxPoolToAddMod-r12ProseDiscTxPoolToAddMod-r12 ::= SEQUENCE { poolIdentity-r12ProseTxPoolIdentity-r12, pool-r12 ProseDiscResourcePool-r12 }ProseTF-IndexPairList-r12 ::= SEQUENCE (SIZE (1..maxProseTF-IndexPair-r12)) OF ProseTF-IndexPair-r12 ProseTF-IndexPair-r12 ::=SEQUENCE { discSF-Index-r12 INTEGER (1.. 200) OPTIONAL, -- Need ONdiscPRB-Index-r12 INTEGER (1.. 50) OPTIONAL -- Need ON }ProseHoppingConfigDisc-r12 ::= SEQUENCE { a-r12 INTEGER (1..200), b-r12INTEGER (1..10), c-r12 ENUMERATED {n1, n5} } -- ASN1STOP

The ‘ProseDiscConfig’ of the table 4 defines dedicated configurationinformation for ProSe direct discovery (D2D discovery).

In the table 4, the ‘ProseDiscResourcePool’ may indicate a plurality ofresource pools for D2D discovery and may include configurationinformation for each resource pool. A following table 5 illustrates anexample of ‘ProseDiscResourcePool’.

TABLE 5 -- ASN1START ProseDiscPoolList4-r12 ::= SEQUENCE (SIZE(1..maxProseTxPool-r12)) OF ProseDiscResourcePool-r12ProseDiscPoolList16-r12 ::= SEQUENCE (SIZE (1..maxProseRxPool- r12)) OFProseDiscResourcePool-r12 ProseDiscResourcePool-r12 ::= SEQUENCE {cp-Len-r12 Prose-CP-Len-r12, period-r12 ENUMERATED {rf32, rf64, rf128,rf256,rf512,rf1024}, numRetx-r12 INTEGER (0..3), numRepetition-r12INTEGER (1..50) OPTIONAL, -- Need OR tf-ResourceConfigProse-TF-ResourceConfig-r12, tx-Parameters SEQUENCE { tx-ParametersProse-TxParameters-r12, ue-SelectedResourceConfig SEQUENCE {poolSelection-r12 CHOICE { rsrpBased-r12 Prose-PoolSelectionConfig-r12,random-r12 NULL }, tx-Probability-r12 ENUMERATED {p25, p50, p75, p100}OPTIONAL -- Need OR } OPTIONAL -- Need OR } OPTIONAL, -- Need ORrx-Parameters-r12 SEQUENCE { tdd-Config-r12 TDD-Config OPTIONAL, -- NeedOR sync-ConfigIndex-r12 INTEGER (0..15) } OPTIONAL, -- Need OR ... }Prose-PoolSelectionConfig-r12 ::= SEQUENCE { threshLow-r12RSRP-RangeProse10-r12, threshHigh-r12 RSRP-RangeProse10-r12 } --ASN1STOP

The ‘ProseDiscPoolList4’ is a list including the number of‘ProseDiscResourcePool’ corresponding to the number of ‘maxProseTxPool’,and defines resources related to transmission of a D2D discovery signal.The ProseDiscPoolList16’ is a list including the number of‘ProseDiscResourcePool’ corresponding to the number of ‘maxProseRxPool’,and defines resources related to reception of the D2D discovery signal.Each ‘ProseDiscResourcePool’ may include a ‘period’ field indicating aperiod where a resource pool is repeatedly indicated.

The UE selects a specific resource for the D2D operation from aplurality of resources indicated by D2D configuration information. Inthis case, the specific resource may be selected in a specific schemeindicated by the D2D configuration information. To this end, the D2Dconfiguration information may include a field indicating a scheme ofselecting a resource for the D2D operation by the UE. For example, inthe table 5, the ‘poolselection’ may indicate one of an ‘rsrpbased’scheme or a ‘random’ scheme.

The ‘rsrpbased’ scheme is a scheme which determines the specificresource based on the RSRP received from the network by the UE. The‘random’ scheme is a scheme which selects the specific resource from aplurality of resources by the UE.

The UE may determines whether the UE is located within coverage of thenetwork or outside of the coverage of the network based on the RSRPreceived from the network.

Meanwhile, the D2D configuration information may include a thresholdvalue with respect to the RSRP. One or more threshold values may begiven and may be used as a reference value with respect to the RSRP. Forexample, the D2D configuration information may include a first thresholdvalue and a second threshold value. The first threshold value may be alow RSRP value THRES_low. The second threshold value may be a high RSRPvalue THRES_high. The above threshold values may be used to determinewhich range the UE is located with respect to coverage of a specificcell transmitting the reference signal.

A coverage range of each UE identified according to the threshold valuemay be related to a specific D2D transmission resource. For example, itis assumed that a coverage range of the UE is divided into three rangesaccording to a plurality of threshold values. It is assumed that a range1 means coverage with the highest RSRP, a range 3 means coverage havingthe lowest RSRP, and a range 2 means coverage having an RSRP higher thanthat of the range 3 and lower than that of the range 1. Each coveragerange may be expressed as a threshold vale set configured by a lowthreshold value and a high threshold value. Accordingly, the threeranges may be expressed as three sets of threshold values with a lowthreshold value and a high threshold value. In general, N coverageranges of threshold values, and each threshold value set may include ahigh threshold value and a low threshold value. If the high thresholdvalue or the lower threshold value is not signaled, it may be assumedthat a RSRP maximum value is a reference value of the high thresholdvalue, and a RSRP minimum value is a reference value of the lowthreshold value.

In the present invention, it is preferable that the threshold value andthe threshold value set are signaled by a related specific resource whenit is considered that each threshold value or each threshold value setare related to a specific coverage range and an available specificresource is related in a specific coverage range. That is, when thenetwork configures a plurality of resources, for example, a plurality ofresource pools in a UE, as illustrated in a following table, it ispreferable to configure a related threshold value and threshold valueset with respect to each resource pool.

TABLE 6 TX POOL#1 TX Pool#1 Time Frequency information RSRP_THRESHOLDTHRES_HIGH THRES_LOW ... TX POOL#N TX Pool#2 Time Frequency informationRSRP_THRESHOLD THRES_HIGH THRES_LOW

In another method of signaling the threshold value and the thresholdvalue set, a list having a plurality of threshold values or thresholdvalue sets is signaled in the UE. In this case, information regardingwhich specific resource is related to a coverage range divided by eachthreshold value and threshold value set may be configured in the UE aslisted in a following table 7.

TABLE 7 TX POOL#1 POOL ID=1 TX Pool#1 Time Frequency information ... TXPOOL#N POOL ID=N TX Pool#2 Time Frequency information RSRP_THRESHOLD#1THRES_HIGH THRES_LOW ASSOCIATED POOL=1 ... RSRP_THRESHOLD#N THRES_HIGHTHRES_LOW ASSOCIATED POOL=N

The UE may perform a D2D operation with other UE using a selectedspecific resource. In this case, the D2D operation may include one ofthe above D2D discovery and D2D communication.

The UE may perform the D2D operation using a resource identified insideand outside the network coverage (cell coverage).

Hereinafter, a situation according to the present invention and adetailed applied example there will be described.

When the UE is served within the network coverage (that is, when the UEis in a RRC_CONNECTED mode or an RRC_IDLE mode), it is not preferable touse a resource for a purpose which is not authenticated. That is becauseuse of the resource may degrade the performance of the whole system andcause violation of laws/rules.

This is the same in the D2D operation. That is, when the UE performs theD2D operation, the UE should select a resource in an authenticatedresource (resource pool, hereinafter referred to as ‘D2D resource’).Theoretically, all cells may have a D2D resource for the D2D operation.Further, the UE requires to include information on a D2D resource in anidle mode. Accordingly, D2D configuration information reporting a D2Dresource for the D2D operation may be broadcasted.

The UE may receive a D2D source pool of a serving cell throughbroadcasted D2D configuration information. Alternatively, the UE mayreceive a D2D resource pool of a serving cell through a dedicate signalthereof.

Further, the UE may receive a D2D resource pool of at least oneneighboring cell. The UE may receive a D2D resource pool of aneighboring cell through broadcasted configuration information.Alternatively, the UE may receive a D2D resource poll of a neighboringcell through a dedicated signal thereof. The D2D configurationinformation may indicate a D2D resource pool configured by cells.

Meanwhile, the network may report a D2D resource for a D2D operation ofanother cell to UEs located within coverage thereof. The above isperformed so that the UEs may receive a D2D signal/message provided fromanother cell. In this case, there may be a problem that the UEs shouldlimit a D2D resource which is used in the D2D operation according to aD2D resource of another cell.

In this case, one of three approaches may be taken into consideration.

1) When a specific resource for D2D transmission is selected, only a D2Dresource of a serving cell of the UE may be considered. That is, the UEmay not consider a D2D resource of a neighboring cell.

2) When a specific resource for D2D transmission is selected, the UE mayconsider a D2D resource of a serving cell of the UE and all availableD2D resources of neighboring cells.

3) When a specific resource for D2D transmission is selected, the UE maybasically consider a D2D resource of a serving cell of the UE and mayselectively consider D2D resources of neighboring cells.

Meanwhile, the UE may receive at least one threshold value with respectto signal strength/quality (e.g., RSRP). The UE compares signalstrength/quality of a serving cell with the threshold value to determinea range of an RSRP. As a result, the range of an RSRP may represent ageographic range related to cell coverage. That is, a great RSRP meansthat the probability of the UE to be located at a center of the cellcoverage is great. A small RSRP means that the probability of the UE tobe located outside the cell coverage is great. Accordingly, a geographicrange may be identified according to a value of the RSRP.

FIG. 18 illustrates cell coverage according to the range of the RSRP.

Referring to FIG. 18, a range A 181, a range B 182, and a range C 183may be determined according to the value of the RSRP. The range A 181may be a region where the value of the RSRP is greater than a firstthreshold value. A region between the range A 181 and the range B 182may be a region where the value of the RSRP is less than the firstthreshold value and greater than a second threshold value (firstthreshold value>second threshold value). An outside of the range B 182may be a region where the value of the RSRP is less than the secondthreshold value. A region having the value of the RSRP less than thesecond threshold value may be regarded as an outside of the cellcoverage.

That is, the network may provide a plurality of threshold values so thatcoverage of a serving cell may be divided into a plurality of ranges.Two continuous ranges may be divided based on a reference value.

Meanwhile, the network may configure an association relationship betweenthe range of the RSRP and a tier and an association relationship betweenthe tier and a cell through D2D configuration information. Since eachcell has a D2D resource thereof, the network associates the range of aRSRP with a D2D resource of each cell through a tier.

For example, a network may provide two threshold values with respect toa cell #1 being a serving cell to report an associated tier with respectto three ranges divided according to each threshold value. Further, thenetwork may report an associated tier with respect to cells #2 and #3being a neighboring cell. If a threshold value is provided toneighboring cells #2 and #3 so that the neighboring cells #2 and #3 maybe divided into a plurality of ranges to report a tier associated withthe divided ranges, respectively. D2D resources are configured in theneighboring cells #2 and #3, respectively, and the D2D resources may beassociated with the tier.

The UE measures a reference signal of a serving cell to obtain an RSRP,and compares the RSRP with a threshold value to determine in which rangeof a serving cell the UE is included. In addition, a tier associatedwith the obtained range is determined using D2D configurationinformation. Accordingly, the UE may know a range in the serving cell inwhich the UE is included and a tier. Next, when the UE determines a D2Dresource for the D2D operation, the UE takes into consideration only aD2D resource of a neighboring cell associated with the tier.

That is, when the UE selects a specific resource for D2D transmission,the UE may recognize a range of the RSRP to take into consideration aD2D resource associated with the range of the RSRP. In this case,consideration of the D2D resource may means that only the resource isregarded as a candidate resource for D2D transmission.

In FIG. 18, if the UE is located at the range A, the UE takes intoconsideration a D2D resource associated with the range A as a resource(resource pool) for the D2D operation. If the UE is located at the rangeB, the UE takes into consideration a D2D resource associated with therange B as the resource (resource pool) for the D2D operation. In thesame manner, if the UE is located at the range C, the UE takes intoconsideration a D2D resource associated with the range C as the resource(resource pool) for the D2D operation.

It is assumed that the range A is associated with a tier 0, the range Bis associated with tiers 0 and 1, the range C is associated with 0, 1,and 2, a service cell (refer to cell 0) is associated with a tier 0,neighboring cells 1, 2, 3, 4, and 5 are associated with the tier 1, andneighboring cells 6 to 11 are associated with the tier 2.

In this case, the UE may select a specific resource for the D2Doperation, a UE located in a range A by taking into consideration a D2Dresource of a cell 0, a UE located in a range B by taking intoconsideration a D2D resource of cells 0 to 5, and a UE located in arange C by taking into consideration a D2D resource of cells 0 to 11 asan example of selecting a D2D resource to be used in each range. As anexample of a method of selecting a resource to be used in a real D2Doperation by the UE by taking into consideration one or more associatedD2D resource, when a corresponding D2D operation is transmission, the UEperforms D2D transmission using an intersection of the one or moreassociated D2D resource. When a corresponding D2D operation isreception, the UE performs D2D reception using a union of the one ormore associated D2D resources. It is assumed in the above example thatthe UE receives signaling of information on a D2D resource of anassociated cell from the network.

As another example of selecting a D2D resource to be used in each rangeby the UE, a UE located in the range A may select a D2D resourceconfigured in the UE by taking into consideration a cell 0, a UE locatedin the range B may select a D2D resource configured in the UE by takinginto consideration cells 0 to 5, and a UE located in the range C mayselect a D2D resource configured in the UE by taking into considerationcells 0 to 11. In the example, the UE does not need to signal cellinformation associated with each coverage range in the UE. The networkneeds to signal only D2D resource information associated with acorresponding range in the UE.

That is, cells may be grouped through a tier. Each group may beassociated with at least one RSRP range. When the UE performs a D2Doperation based on a serving cell, the UE may identify a RSRP range(accordingly a located range of the UE) through measurement of a servingcell. When the UE performs the D2D operation based on a cell which isnot the serving cell, the UE identifies a RSRP range (accordingly alocated range of the UE) through measurement of a reference cell toselect a D2D resource to be used by the UE.

Meanwhile, when a D2D resource is selected for D2D transmission, the UEmay take into consideration a D2D resource of a serving cell or a D2Dresource which is not associated with the serving cell. That is, the UEmay take into consideration a D2D resource of the serving cell or a D2Dresource of a virtual cell which is absent.

A network may configure an association relationship between a range of aRSRP and a tier and an association relationship between the tier and acell. In this case, each cell may include a serving cell and a virtualcell. Moreover, since a D2D resource may be configured in each virtualcell, a range of an RSRP is finally associated with a D2D resource ofthe serving cell or the virtual cell through a tier.

For example, the network may provide two threshold values with respectto a cell #1 being a serving cell to report a tier associated with threeranges divided by each threshold value. In addition, the network mayreport an associated tier with respect to a virtual cell. In this case,the virtual cell may indicate coverage outside of the serving cell, andindicates a concept of the virtual cell for convenience. If a thresholdvalue is provided to the virtual cell so that the virtual cell isdivided into a plurality of ranges, the network may report a tierassociated with the divided ranges, respectively. A D2D resource isconfigured in the virtual cell. This may be interpreted to indicate thata D2D resource to be used outside the coverage of the serving cell.

The UE measures a reference signal of a serving cell to obtain an RSRP,and compares the obtained RSRP with a threshold value to obtain in whichrange of the serving cell the UE is included. Moreover, a tierassociated with the obtained range is determined using the D2Dconfiguration information. Accordingly, the UE may know a range to whichthe UE is included and a tier in a serving cell. Next, when the UEdetermines a D2D resource for the D2D operation, if the UE is includedin a specific range of the serving cell and a specific tier associatedwith the specific range, the UE may take into consideration only a D2Dresource of a virtual cell associated with the specific tier. Forexample, a range of the serving cell is divided into an inside and anoutside of the serving cell according to one threshold value. The insideof the serving cell is associated with a tier 0 and the outside of theserving cell is associated with a tier 1. Since a D2D resource isconfigured with respect to the virtual cell if the outside of theserving cell is a virtual cell, a D2D resource is configured in theoutside of the serving cell. If the virtual cell is associated with atier 1, the UE determines a range based on a RSRP in the serving cell.If the range is determined as the inside of the serving cell, the UEdetermines a resource to be used in a real D2D operation by taking intoconsideration only a D2D resource configured in the serving cell.Further, if the range is determined as the outside of the serving cell,the UE determines a resource to be used in a real D2D operation bytaking into consideration only a D2D resource of the serving cellassociated with a tier 1.

That is, when the UE selects a specific resource for D2D transmission,the UE recognizes a range of an RSRP to take into consideration a D2Dresource associated with the range of a RSRP. In this case, the UE takesinto consideration a D2D resource of a serving cell or a D2D resource ofa virtual cell which is not the serving cell.

FIG. 19 illustrates the relationship between a range of an RSRP and atier and the relationship between the tier and a cell.

Referring to FIG. 19, a range A is associated with a tier 0, and thetier 0 is associated with a cell 0. In this case, the cell 0 may be aserving cell.

A range B is associated with a tier 1 and the tier 1 is associated witha cell 1. The cell 1 is a virtual cell. A range C is associated with atier 2 and the tier 2 is associated with a cell 2. The cell 2 is alsothe virtual cell.

Resource pools #0, 1, and 2 are sequentially configured in the cells 0,1, and 2 as a D2D resource, respectively.

In this case, if the UE is located in the range A, the UE performs a D2Doperation using a resource pool #0. If the UE is located in the range B,the UE performs a D2D operation using a resource pool #1. Furthermore,if the UE is located in the range C, the UE performs the D2D operationusing a resource pool #2.

That is, when the UE is located within coverage of a reference cellperforming D2D, the UE performs a D2D operation using a resourceassociated with a current coverage range of the reference celldetermined by the UE among D2D resources of the reference cell. When theUE is located outside the coverage of a reference cell, the UE performsthe D2D operation using a preset different D2D resource. According tothe present invention, the above is described by applying a concept ofthe virtual cell. The UE receives/measures a reference signal from aserving cell to obtain a RSRP value and may know in which range the UEis included. If the measured RSRP value is lower than a specificthreshold value, the UE may determine that the coverage is beyond thecoverage of the serving cell.

Meanwhile, all cells have the above same threshold value or a servingcell may have a threshold value different from that of a neighboringcell. Further, the threshold value may include a default thresholdvalue. The default threshold value may be the lowest threshold value ofthe serving cell. If the UE receives the above threshold value, the UEcompares a signal strength/quality (RSRP) of a cell with the thresholdvalue. If the measured signal strength/quality of a cell exceeds thethreshold value, the UE takes into consideration a D2D resource of thecell. Otherwise, the UE does not take into consideration a D2D resourceof the cell. That is, when a specific resource is selected for the D2Doperation, a D2D resource of each cell may be considered or notconsidered as an ON/OFF equation. For example, it is assumed that the UEreceives a threshold value with respect to cells #1, #2, and #3. In thiscase, the

#1 is a serving cell, and the cells #2 and #3 are a neighboring cell.The UE may measure a reference signal with respect to the cells #1, #2,and #3 to obtain an RSRP, and may compare a RSRP with respect to eachcell with a received threshold value with respect to each cell. As aresult, if only RSRPs of the cells #1 and #2 are greater thancorresponding threshold values, the UE takes into consideration only D2Dresources of the cells #1 and #2 upon selection of a resource for theD2D operation. That is, a D2D resource of the cell #3 is not considered.That is, when a specific D2D resource is selected for the D2D operation,it is determined whether to use (consider) a D2D resource of each cellaccording to signal strength/quality and a threshold value of each cell.

The UE is allowed to use a specific resource in a resource definedaccording to D2D configuration information upon D2D transmission. The UEis not allowed to use a resource except for a resource indicated by D2Dconfiguration information. The UE may know a resource which may have D2Dtransmission of another UE.

FIG. 20 illustrates an example of applying a method described withreference to FIG. 17 to FIG. 19.

Referring to FIG. 20, a network transmits D2D configuration informationto UE 1 (S150). Corresponding D2D configuration information includesinformation indicating a plurality of D2D resources. For example, thecorresponding D2D configuration information may include informationindicating a plurality of D2D transmission pools. Moreover, anindividual D2D resource, for example, an individual transmission poolmay include threshold value information indicating a reception powerrange (e.g., RSRP) to which a corresponding pool is applicable. Next,the network transmits a reference signal to the UE 1 (S151). Thereference signal is a signal for measuring reception quality of a cellto which UEs transmit the reference signal. The UE 1 receives areference signal to measure reception power RSRP (S152).

The UE 1 selects a resource for the D2D operation (S153). For example,the UE 1 selects a D2D transmission resource (e.g., transmission pool)to which the measured reference signal is applicable.

The UE 1 performs a D2D operation with the UE 2 using the selectedresource (S153).

FIG. 21 is a block diagram illustrating a terminal according to anembodiment of the present invention.

Referring to FIG. 21, a terminal 1100 includes a processor 1110, amemory 1120, and an RF unit 1130. The processor 1110 performs theproposed functions, processes and/or methods. For example, the processor1110 receives D2D configuration information indicating a plurality ofresources to be used in the D2D operation to select a specific resourcefrom the plurality of resources. In this case, the terminal selects thespecific resource based on reception power RSRP of a reference signalreceived from the network. Next, the processor 1110 performs the D2Doperation with another terminal using the selected specific resource.

The RF unit 1130 is connected to the processor 1110 and sends andreceives radio signals.

The processor may include Application-Specific Integrated Circuits(ASICs), other chipsets, logic circuits, and/or data processors. Thememory may include Read-Only Memory (ROM), Random Access Memory (RAM),flash memory, memory cards, storage media and/or other storage devices.The RF unit may include a baseband circuit for processing a radiosignal. When the above-described embodiment is implemented in software,the above-described scheme may be implemented using a module (process orfunction) which performs the above function. The module may be stored inthe memory and executed by the processor. The memory may be disposed tothe processor internally or externally and connected to the processorusing a variety of well-known means.

What is claimed is:
 1. A method for transmitting a device-to-device(D2D) signal in a wireless communication system, the method performed bya user equipment (UE) and comprising: receiving, from a serving cell,D2D configuration information informing a plurality of resource poolswhich can be used for transmission of a D2D signal; and transmitting theD2D signal based on a resource pool among the plurality of resourcepools, wherein the D2D configuration information includes informationinforming whether the resource pool is selected based on a referencesignal received power (RSRP) or is selected randomly, wherein, based onthe resource pool being selected based on the RSRP, the resource pool isa resource pool having a measured RSRP value that is larger than a lowthreshold value and smaller than a high threshold value, and wherein thelow threshold value and the high threshold value are informed by the D2Dconfiguration information.
 2. The method of claim 1, wherein the D2Dconfiguration information informs the low threshold value and the highthreshold value such that the UE selects only one resource pool upon theRSRP based pool selection.
 3. The method of claim 1, wherein the D2Dsignal comprises a signal for a D2D discovery.
 4. The method of claim 1,wherein the D2D signal comprises a signal for a D2D communication.
 5. Auser equipment (UE), the UE comprising: a transceiver configured totransmit and receive radio signals; and a processor connected to thetransceiver to be operated, wherein the processor: receives, from aserving cell, D2D configuration information informing a plurality ofresource pools which can be used for transmission of a D2D signal; andtransmits the D2D signal based on a resource pool among the plurality ofresource pools, wherein the D2D configuration information includesinformation informing whether the resource pool is selected based on areference signal received power (RSRP) or is selected randomly, wherein,based on the resource pool being selected based on the RSRP, theresource pool is a resource pool having a measured RSRP value that islarger than a low threshold value and smaller than a high thresholdvalue, and wherein the low threshold value and the high threshold valueare informed by the D2D configuration information.
 6. The UE of claim 5,wherein the D2D configuration information informs the low thresholdvalue and the high threshold value such that the UE selects only oneresource pool upon the RSRP based pool selection.
 7. The UE of claim 5,wherein the D2D signal comprises a signal for a D2D discovery.
 8. The UEof claim 5, wherein the D2D signal comprises a signal for a D2Dcommunication.